The present invention relates to a method and apparatus for the ultrasonic characterization of polymers under simulated processing conditions. More particularly, the invention is directed to an improved buffer rod technique enabling the complete characterization of polymer materials at temperature and pressure conditions normally encountered in actual processing.
Processing of polymers includes heating, melting, applying pressure and cooling, all at certain rates under given conditions of flow geometry. The corresponding parameters (temperature, temperature gradient, pressure and time) will affect the flow properties during processing and thus influence the quality of the end product.
When considering polymer processing, it is important to distinguish between two classes of synthetic polymers: the thermosets and the thermoplastics. The thermosets are those where a crosslinking agent (hardener) is added to a low molecular weight liquid (resin); polymerization is carried out in a mold and yields a stable end product. The process is irreversible. The second class is that of thermoplastics, more commonly referred to as plastics; the term "plastics" comes from the fact that the resin material is heated until pliable and soft enough to be formed into the desired shape. In contrast to thermosets, thermoplastics are high molecular weight polymers which preserve their chemical and structural identity during processing.
Basically, polymers are constituted of molecules comprising a large number of repeating units which are covalently linked together to form chains. In turn, the attraction between the chains is provided by secondary bonds which, depending on the nature of the molecules, may be Van-der-Waals forces, dipole-dipole interactions, hydrogen bonds and, if crosslinking is present, covalent bonds. At the higher temperatures, typically 200.degree.-300.degree. C., the configuration for the chains is random, like that of a viscous liquid. The specific volume (V) of this liquid can be described as the sum of a free volume (V.sub.f) associated with holes and packing irregularities, and that of an occupied volume (V.sub.o) corresponding to the effective volume occupied by the molecules under Brownian agitation. As temperature (T) is lowered, the thermal excitation diminishes and this is accompanied by a decrease of the specific volume (.alpha.=d(ln V)/dT.perspectiveto.10.sup.-3 /deg), mainly due to the decrease of free volume (V.sub.f). This collapse of V.sub.f limits the freedom of movement for the chains, which results in the increase of viscosity (.eta.). Upon approaching a certain characteristic temperature, referred to as the glass transition temperature (Tg), the free volume has so diminished that the possibility of conformational rearrangements of the chains has become strongly hindered. In this region of temperature, the response to a perturbation is characterized by a finite relaxation time, .tau.(10.sup.3 to 10.sup.-9 sec), so that material properties such as moduli and viscosity are also time and frequency dependent. The time or frequency dependent nature of material properties is a characteristic of polymers and is referred to as viscoelasticity. On going through Tg, the glass transition temperature, the thermal expansion suddenly drops (.alpha..perspectiveto.10.sup.-4 /deg) and the viscosity increases to such values (.eta..perspectiveto.10.sup.13 poises) that the structure is frozen in, and the material appears as a solid. Now, because of its high viscosity, the liquid may be undercooled to various degrees so that the value for Tg is dependent on the cooling rate. Therefore, the structure and properties of the solid are contingent not only upon the nature of the molecules, but also, to a large extent, upon the thermal history near Tg, i.e., temperature, pressure and cooling rate.
According to their structure, solid polymers are found to be either amorphous or semi-crystalline. Amorphous polymers, such as the usual atactic polystyrene (PS) and polymethyl-metacrylate (PMMA or plexiglass), are characterized by the absence of long range ordering of the molecules. Semi-crystalline polymers, such as polypropylene (PP) and polyethylene (PE), are those where the formation of ordered regions (crystals) dispersed in the amorphous matrix is favoured by strong intermolecular forces and by the highly regular structure of the chains. In the case where such ordering exists, the semi-crystalline material still exhibits a glass transition temperature (Tg) which is related to the amorphous nature of the matrix, but also a melting point (Tm) and a solidification point (Ts) which are related to the crystalline phase.
Polymeric materials can be formed by a variety of fabrication techniques. The major component in most processing lines is the extruder. The raw polymer in pellet or powder form is fed from a hopper to a rotating screw. The material is driven through a cylinder where it is heated, compacted and softened. In the simple extrusion process, the screw forces the material through a die of a given shape and the shaped extrudate (tubing, pipe, fiber, sheet, insulating wire coatings . . . ) is then cooled to solidification. In blow molding, a tube is first extruded and while the material is still in the molten state, an air jet inflates the tube to the desired form, either inside a mold (blow molding) or as thin-wall oblong ballon (blow film extrusion). In injection molding, a charge of molten material is first accumulated in the nozzle of the extruder before it is injected in a cold mold. In rotational molding, a powder is placed into a cold mold and the mold is then rotated as it is heated; this causes the polymer to soften, melt and coat the inside of the mold to form a hollow object. Calendering of thermoplastics involves feeding the polymer between a series of heated rollers that flatten, extend and draw the compound into a sheet or film. In the thermoforming process, a thermoplastic sheet is first heated to its softening point, then shaped into a die by either a mechanical force, air pressure or vacuum. Compression molding (or transfer molding) is one where a press with heated platens is utilized to shape the material into a hot mold. The above methods are those which are typically used for thermoplastics; however, in many cases, they can also be used for processing thermosets, e.g., injection molding, rotational molding and compressional molding.
From what has been said above about the nature of the polymer material and its processing, it can be foreseen that the incentive for methods of testing both the end use product and the raw material is very strong. Indeed, a number of analytical techniques have been developed which can be classified as either reliability or thermostructural tests. Reliability tests are made under conditions that approximate the end use environment and serve to predict long term stability; such tests include time dependence of compliance, creep, time to rupture, brittleness, hardness, resistance to abrasion, wear, friction, and fatique, electrical resistivity and strength.
The characterization of the raw material, on the other hand, is made with the help of thermostructural tests which aim at correlating structural changes that occur within the polymer wich changes in a thermodynamic property; they include specific heat, thermal conductivity and expansion, degradation, static and dynamic mechanical analysis, dielectric behaviour, I.R., Raman and N.M.R. spectroscopy.
For a given application, one technique may prove to be more adequate, but none is universal. Moreover, the above mentioned techniques are often difficult to implement and are time consuming; also, they involve delicate and costly instrumentation and require skilled technicians. Therefore, quality control laboratories often rely on tests which have been elaborated on common sense basis, such as melt flow index, capillary flow and softening point index. Such tests are often very useful but they usually involve a combination of a great number of physical parameters which makes them difficult to interpret in terms of more fundamental and basic material properties. As a consequence, the results obtained from such tests are specific to the test conditions and cannot be extrapolated to different conditions; the results are indicative and not quantitative.
As previously mentioned, in the course of being processed, the polymer material is subjected to a number of different thermomechanical conditions which involve temperature, pressure and time. Given that the properties of the final end use material are highly dependent upon its past thermal history, it would be highly desirable to have means of characterizing the raw polymer under conditions that are representative of those which it will encounter during the processing operations, giving access to the fundamental material properties.
In this perspective, ultrasonic methods for the testing of polymers have been developed using either liquid immersion techniques, solid buffer rod techniques and direct contact techniques.
Ultrasonic immersion techniques are rather well known, whereby a sample is placed in the path of a sound beam between a transmitting transducer and an opposite receiving transducer which are immersed in a sound conducting fluid. Changing the orientation of the sample with respect to the sound path allows, under certain conditions, to generate longitudinal waves (when the sound beam is perpendicular to the plane of the sample) or shear waves (when the angle of incidence exceeds a critical value for total internal reflection, according to Snell's law). A method has been described in H. A. Waterman, Kolloid-Zeitscrift and Zeitschrift Fur Polymere, 192,1 (1963) that allows measurements to be made on solid materials only. The method precludes testing of polymers at higher temperatures because the material may be deformed. The approach was modified as shown in U.S. Pat. No. 3,858,437 to Jarzynski et al. and in the article by B. Hartmann and J. Jarzynski, J. Acoust. Soc. Am. 56, 1469 (1974), whereby the sample is held stationary in a horizontal position and the transducer assembly itself is rotated around the sample. In this technique, experiments must be carried out on a number of specimens of different thicknesses such that the method is restricted to temperatures where the attenuation is not so high as to prohibit the transmission of sound through the thicker samples.
U.S. Pat. No. 4,346,599 to McLaughlin et al. and the article by G. W. Paddison, Proc. IEEE Ultrasonics Symposium, 502 (1979) describe an improved version of the technique. In particular, the authors of these references point out to the necessity of measuring the thickness (1) and the density (.rho.) or specific volume (V=1/.rho.) at each of the temperatures at which acoustic measurements are made. The authors rightfully point out that polymers have a mass density with a temperature dependence which is important and must be accounted for in the determination of the storage and loss moduli. Furthermore, the authors call attention to the fact that the density of polymers may depend on the rate of heating.
In its application to studies on polymers, the immersion technique, however sophisticated, suffers some inherent limitations with respect to temperature and pressure. In order for a given method to be versatile, it should allow measurements to be performed in as broad a temperature range as possible, while the immersion technique allows to cover only a limited zone below the boiling point of the sound transmitting liquid where the viscosity of the liquid is still reasonably small. Also, the properties of polymers being strongly temperature dependent, an accurate measurement of temperature is required, which is difficult to obtain with the immersion method; unless care is taken to stabilize the temperature of the liquid, thermal gradients will exist which will perturbate the temperature of the sample. Also, the properties of polymers are influenced by the rate of temperature change. In order to permit the study of these effects, the technique should allow a close control of heating or cooling rates over as broad a range as possible. This is not possible using an immersion apparatus, given its high thermal inertia (transducer assembly, immersion fluid, etc.). Another important restriction stems from the fact that the immersion liquid will very often chemically react with the polymer, especially at high temperatures, and this will invalidate all results. Finally, during processing, the molten polymer will also experience very dramatic pressure changes (0-2 KBars) and an experimental method is required which allows to study the influence of pressure; the immersion technique cannot be easily adapted to studies under such conditions.
The immersion technique also suffers from some technical drawbacks. As can be seen in U.S. Pat. No. 4,346,599, the apparatus requires a great number of mechanical and moving parts (e.g. motors, gears, chains, cams, etc.) which demand minute adjustments. Furthermore, mechanical tunings must be made before any measurement (e.g. align sample, rotate transducers and find normal incidence conditions, rotate sample and find critical angle for extinction). Such tunings are critical and could lead to erroneous results if not performed properly. As such they constitute a source of measurement error. Also, they must be performed manually by an operator and because of this, the immersion technique cannot be made fully automatic.
Ultrasonic methods using solid buffer rod techniques are known. In such techniques, the sample is placed between two solid (metal or glass) rods. A first transducer acting as a transmitter is attached to the free end of one rod and a second transducer, the receiver, is attached to the free end of the other rod. The transmitter produces a burst of ultrasound which travels through the first rod, part of the energy being transmitted through the sample where it interacts with the material. Then, the ultrasound impinges on the second buffer rod and part of the energy is directed along the rod to the receiver where it is detected. Reference to such methods were made in the aforementioned McLaughlin et al. patent and can also be found in the review article by H. J. McSkimin, "Ultrasonic Methods for Measuring the Mechanical Properties of Liquids and Solids" in "Physical Acoustics", edited by W. P. Mason, Academic Press, New York (1964), Vol. I-A, Chap. 4, pp. 271-334.
All of the aforementioned techniques suffer a common most important drawback in that none provide the simultaneous measurement of density (.rho.) or specific volume (V=1/.rho.). In the technique described by J. R. Asay, D. L. Lamberson and A. H. Guenther, J. Appl. Phys., 40,4 (1969) and D. L. Lamberson, J. R. Asay and A. Guenther, J. Appl. Phys., 43,3 (1972), the thickness of the polymer sample is estimated indirectly by use of an ad hoc calculation, which involves a great deal of uncertainty. In all the other techniques, a fixed length spacer is placed between the buffer rods, which determines the length of the sample. Under such conditions, either the sample is confined and cannot expand nor contract freely such that the pressure conditions are unknown, or the sample is allowed to leak out of the cavity such that its mass is unknown, which makes impossible the determination of density.
As previously stated, an experimentation carried out on polymer materials requires close control of thermal history (value of temperature, prescribed heating or cooling rates) over extended ranges. However, the above mentioned techniques are restricted to limited temperature ranges (usually between 20.degree. and 100.degree. C.) and furthermore make no provision to control the heating and/or cooling rates. Measurements are usually carried out either at fixed temperatures or at a heating or cooling rate which is determined by the thermal inertia of the apparatus. Moreover, none of these techniques are designed to allow measurements under varying and controlled pressure conditions.
The buffer rod technique is useful in the case where high attenuation in the material under investigation imposes the use of short samples. In such situations, the buffer rods serve to circumvent problems associated with reverberation of sound by providing a time delay. However, other methods are available whereby the transducers are directly bonded to the sample (direct contact techniques).
An application of the technique to measurements under pressure is described in Asay et al. and also in Lamberson et al. In this application, the transducer-sample assembly is contained in a high pressure vessel and immersed in a pressure transmitting fluid; a furnace, enclosed in the pressure vessel, allows to heat the sample. For utilization in studies on polymers, such a design suffers the same drawbacks as those mentioned above, concerning the immersion technique and others.
The first drawback comes from that the temperature is limited to within the range specified for the hydraulic fluid and pressure vessel. Secondly and most importantly, because of the high thermal inertia, the heating and cooling cannot be controlled and the apparatus is limited to measurements performed at preset values of temperature. Thirdly, there arises the possibility that the sample be degraded by the hydraulic fluid through chemical reaction. Finally, the technique is laborious to operate as it involves making new bonds between the transducer and the sample for each different sample; the instrument is not easy to implement as it involves a number of technical difficulties associated with leakage of the hydraulic fluid.